EPO publication 0 207 811 A3 equivalent of U.S. Pat. No. 4,990,2368 discloses substantially crystalline, monodispersed, monolayered mixed metal layered hydroxides (MMLH) of the general formula EQU Li.sub.m D.sub.d T(OH)(m+2d+3+n.multidot.a)(A.sup.n)a.multidot.qH.sub.2 O(I)
where:
m is from zero to about 1; PA1 D represents divalent metal ions; PA1 d is from zero to about 4; PA1 T represents trivalent metal ions; PA1 A represents monovalent or polyvalent anions or negative-valence radicals other than OH-ions; PA1 a is the number of ions of A; PA1 n is the valence of A; PA1 n.multidot.a is from zero to about -3; PA1 q is zero or more; PA1 (m+d) is greater than zero; and PA1 (m+2d+3+n.multidot.a) is equal to or greater than 3.
In the above formula, n.multidot.a means n multiplied by a, and the product of n.multidot.a is either zero or is negative.
For purposes of brevity, the expression MMLH will be used in this disclosure as an acronym for the mixed metal layered hydroxides which conform essentially to the generic formula shown above. In some cases other, more specific, identification of compounds will also be used in appropriate places.
Stated briefly, the preparation of the above-described MMLH compounds involves mixing together soluble compounds of the desired metals in the desired ratio and then, in a continuously flowing, steady state, flash coprecipitation technique, reacting the mixture with a source of OH-ions, such as NH40H, to produce the substantially crystalline, monodispersed, monolayered MMLH compounds within the generic formula shown above. Further details of the process are available in the above-identified EPO application or in pending U.S. application Ser. No. 0,60,133, filed Jun. 9, 1987 (U.S. No. 4,990,268) which is related to the above-identified EPO application. Another patent, U.S. Pat. No. 4,790,954, discloses MMLH compounds as adducts with clay, such as bentonite, which are thickeners for water and other hydrophylic fluids. The references disclose the crystal thickness of the monodispersed, monolayered compounds as being in the 8 to 16 angstrom range, which is extremely small for a mixed metal crystal. The above patents and applications are incorporated herein by reference.
Fumed silica is well-known in industry as a highly pure, very fine particle size silica, SiO.sub.2, which is usually made from SiCl.sub.4, but can be made from SiF.sub.4, e.g., in accordance with U.S. Pat. No. 4,059,680. Fumed silica particles are so fine, that they are sometimes referred to as "silica smoke". In this disclosure, the expression "FS" is used to signify "fumed silica".